Polymeric foams can be generally classified as either closed-cell foams or as open-cell foams. Open-cell foams can be used as a matrix to contain various liquids and gases. They are capable of various industrial applications such as, for example, use in wipes and diapers, as carriers and ion exchange resins. For some of these applications, it is desirable to have porous crosslinked polymer blocks which have a very low density and a high capacity of absorbing and retaining liquids. Such high absorption capacity, low density, porous polymer blocks can be prepared by polymerizing a specific type of water-in-oil emulsion known as high internal phase emulsion (HIPE) having relatively small amounts of a continuous oil phase and relatively greater amounts of an internal water phase.
Such high absorption capacity, low density foams are prepared in U.S. Pat. No. 4,522,953 by polymerizing and crosslinking the monomers in the continuous oil phase of a high internal phase water-in-oil emulsion with a polymerization initiator such as potassium persulfate. Generally, these high internal phase water-in-oil emulsions contain at least 90 weight percent of an aqueous liquid as the internal phase. The high ratio water-in-oil emulsions are formed by combining the oil phase with water under moderate shear. In order to obtain this high internal phase water-in-oil emulsion, a surfactant must be used to stabilize the inverse emulsion.
One class of surfactants used to produce foams by such processes are sorbitan fatty acid esters. Commercial sorbitan fatty acid esters are a combination of mono-, di-, tri-, and tetra-fatty acid esters (generally C.sub.8 -C.sub.18) of sorbitan C.sub.6 (H.sub.2 O).sub.5 H.sub.2, as well as mono- and di-fatty acid esters of isosorbide C.sub.6 (H.sub.2 O).sub.4 H.sub.2 and polyol impurities.
The stability of the emulsion is directly tied to the surface activity of the emulsifier. A robust emulsifier will stabilize emulsions up to high (30:1) water:oil ratios. A poor emulsifier will not, and at high water to oil ratios the emulsion will degrade by not incorporating water, and eventually break completely.
One method of enhancing the emulsifier is to combine different sorbitan fatty acid esters as reported in U.S. Pat. No. 5,200,433. However, it is desirable to further enhance the emulsion stability to obtain higher ratio water-in-oil emulsion increasing the overall absorption capacity.
It is therefore an object of the present invention to provide a more effective surfactant system for the preparation of a high internal phase water-in-oil emulsion for the preparation of a low density crosslinked polymeric material.